As is well known, epoxy resin compositions have exellent mechanical properties such as excellent tensile strength and flexural strength and also excellent electrical properties such as excellent dielectric properties and volume resistivity, and hence, the compositions have widely been used in various utilities, such as adhesive materials, coating materials, insulating materials for various electric and electronic devices.
However, the epoxy resin of the main component in these compositions has usually inferior in crack resistance and toughness and further it is difficult to relax the stress due to shrinkage when cured. In order to improve these drawbacks, there have been proposed various modifications of the composition. For example,
(1) it is incorporated with specific epoxy resins, such as polyethylene glycol diglycidyl ether, hexanediol diglycidyl ether, urethane-modified epoxy resin, thiocolmodified epoxy resin, dodecenylsuccinic anhydride, polyazelaic polyhydride, and the like, in order to give flexibility to the composition, or
(2) particles of rubber having low glass transition temperature (Tg) such as modified acrylonitrile-butadiene rubber or acrylic rubber are dispersed therein.
According to the method of the above (1), however, the resulting composition has slower curing speed, lower heat resistance and further the cured product thereof loses its flexibility with lapse of time. Besides, according to the method of the above (2), the resin has undesirable increased water absorption.